Abstract: Abstract: Soil free hydrocarbon technique by directly detecting free light hydrocarbon anomalies affected by the landscape medium of little influence is introduced to obtain underground oil information in this paper. The known petroleum reservoir has a good show of free hydrocarbon geochemical anomalies in the test area in Tarim Basin, the aridity coefficient between 840 and 1989 of which indicates that free hydrocarbon geochemical anomalies is related to the thermal evolution of deep oil and gas. The underground oil and gas reservoir properties predicted and judged by the balance coefficient, soil humidity coefficient and characteristic coefficient of free hydrocarbon component are consistent with the actual geological results. The average of CH4/C3H8 index of nearsurface free hydrocarbon from north to south is between 25 and 116,which illustrates that the depth of oil and gas reservoir increases in turn. The feature of the butane heterogeneous ration and pentane heterogeneous ration shows that Yakela petroleum region has the highest maturity of organic matter. The result indicates that the component and the content variation of free hydrocarbon can forecast and judge the location, character, maturity of organic matter and preservation condition of underground oil and gas reservoir.

Key words: free hydrocarbon, reservoir property, preservation condition, maturity, Tarim Basin